Dangerous Hail

Every year, tornadoes, hurricanes, and snowstorms claim lives and cause injuries. But strangely, hailstorms, which pelt the ground with hard balls of ice, never seem to hit human targets. A listener called Science Update to ask why.

Transcript

How people weather hailstorms. I'm Bob Hirshon and this is Science Update.

We've all heard stories about golf ball- and baseball-size hail denting cars and damaging houses. So that prompted Lee Greenfield of Washington, D.C., to ask why we never hear about people getting hurt by hail.

We checked with Harold Brooks, a research meteorologist at The National Severe Storm Lab in Norman, Oklahoma. He says hail stones have to be a couple inches in diameter to cause injury.

Brooks: But hail of that size is pretty rare. And hail that could really seriously hurt you—getting to be baseball-size and larger—there aren't very many storms each year that do that. And even within a storm that does produce hail that size, not very many of the stones are that big. Most of the stones are much smaller than that.

Brooks adds that the worst hailstorms happen in the West, where population density is low, reducing the chances of getting hit even more. That doesn't mean the risk is zero, though.

Brooks: We know that there've been a couple of...small plane crashes that have been associated with hail. And so we know that there've been at least a few fatalities in the United States. So it's not that people don't get hurt or killed by hail, it's just that in industrialized countries it's not very common.

If you've got a science question, hail us at 1-800-Why-Isit. If we use it on the show, you'll get a free Science Update mug. For the American Association for the Advancement of Science, I'm Bob Hirshon.

Making Sense of the Research

This question is really about explaining risk, which is basically the probability that something bad might happen. It's easy to make judgments about risk based on casual observation, personal perception, or anecdotal evidence, but these judgments are often incorrect. For example, far more people are afraid of flying than driving, but driving is actually much riskier (in fact, in the year 2002 there were no fatal accidents among domestic, commercial airlines in the United States, but there were tens of thousands of fatal auto accidents).

Here, the listener's perception—that relatively few people get killed or injured by hail—is correct. So, in other words, your risk of death from hail is pretty low. Why is that? Is it because hail is not usually dangerous, or because it's unlikely to hit you?

As Brooks explains, it's really a combination of both. Hail is rarely big enough to be dangerous, and even if it is, a big chunk is unlikely to knock you out. Brooks mentions that most big hail falls in the underpopulated West, which reduces the probability of human injury. Also, when you compare houses getting damaged by hail to people getting damaged by hail, a couple other key differences come to mind: First of all, houses and buildings are much larger than people; the old cliché "it's as easy as hitting the side of a barn" certainly applies to a real barn. Secondly, during a hailstorm, most people tend to seek shelter, usually inside a house or a car. But the house and the car have nowhere to hide, so they're left to withstand Mother Nature's onslaught.

A similar question that was once answered on Science Update was "Why do tornadoes always head for trailer parks?" The answer was not that tornadoes have a thing against trailers, and it also wasn't true that trailers were more plentiful in tornado-prone states (there are lots of trailers in the Southwest and the Southeast, for example, where tornadoes are rare). Rather, it's just that trailer parks are especially susceptible to tornado damage, since the homes are meant to be lightweight and, well, mobile. Unfortunately for trailer owners, tornadoes don't distinguish between "mobile" and "airborne."

Now try and answer these questions:

Why are hail-related injuries uncommon?

Aside from the reasons cited here, can you think of other reasons why hail doesn't cause many deaths and injuries? For each possible explanation, discuss whether the information presented here supports it, contradicts it, or leaves it unresolved.

Consider the following (hypothetical) statistic:

Last year, the number of people killed by lightning in Pennsylvania was at least double the number killed by lightning in any other state.

Suppose you were hired by the government of Pennsylvania to investigate this situation. What kinds of questions would you ask? What research would you do? Discuss possible explanations and how you would go about testing each one.